nocatsplash is used in the Chinese Tomato DualWan (for which the web files are of course available!). With some additional c -sources provided by The Chinese author we had a go at implementing but it didn't always work, but same issues observed with the DualWan implementation - so I know Victek carried on and looked at nodogsplash / wifidog.

In principle it shouldn't be too difficult - it just manages iptables rules linked with arp-binding, so maybe the issue is integration with Tomato firewall and QOS/access restrictions etc. But this interraction with existing netfilter configuration means it is not something that can be added simply with optware etc.

You really need multiple SSID (private/guest) for this as well - so it doesn't really fit Tomato simple/slick - so I recommend you use dd-wrt.